Hold your jewelry item under running water and rinse off the toothpaste. If your gold piece is real, the black spot will disappear making it look clean and shiny. If you can still see the black spot, this means the item is fake. Do note that this test is not 100% accurate.
Test Gold Using the Float Test
Take a cup or glass, fill it up with water, now bring the gold that you want to test. Drop it into this filled glass. If the gold floats, it is surely not real but if the gold sinks to the end of the glass then it is pure gold. The real gold will sink due to being a heavy metal.
Fill a cup or bowl with water and carefully drop your gold piece into it. If the gold is real, it will sink to the bottom of the cup. If it's fake, it will float to the top or hover in the middle of the cup.
Toothpaste can damage your diamonds, gemstones, gold and silver. Toothpaste is abrasive and has a hardness of around 3/4 on the Mohs Scale of Hardness. Metals such as gold and silver are softer, so toothpaste can actually scratch damage your gold and silver jewellery.
White vinegar is the most acidic vinegar, so it works the best for testing gold. It is also clear-colored, so it will show a color change the best. You'll want to use an eyedropper to pour your vinegar so that you can control the amount that you put on your gold piece.
Acid test.
An acid test can reveal the karat of solid gold jewelry, but it can also show whether jewelry is solid gold or gold-plated. With an acid test, a small sample of the jewelry is removed and exposed to acid to induce a color change. The resulting colors indicate which type of metal the jewelry is made of.
Make a small scratch on the gold and add a drop of bleach. If it changes color, your jewelry is not 100% pure 24K gold. Bleach can permanently damage gold jewelry. It can't tell you if your gold is fake, since most gold jewelry has other metals mixed in to give it strength.
Don't use silver polish, toothpaste, harsh chemical cleaners, or abrasive scrub brushes on gold-plated jewelry.
The toothpaste will not affect real gold, but impure gold will change color. The black streak that appears is due to the presence of other metals, like copper or silver. Keep in mind that this test will not work on gold-plated items. The toothpaste will remove the thin layer of gold, revealing the metal beneath.
In their pure forms, gold and silver happen to be very soft metals—soft enough that you should be able to mark them with your teeth. According to the Mohs hardness scale—which relates pairs of materials according to which one will scratch the other first—gold scores a 2.5 and silver, which is harder, a 2.7.
To perform the test, first, scratch a small area of the gold with the file or sandpaper. This will remove the top layer of gold, exposing the inner layer. Next, apply a small drop of nitric acid to the scratched area. Observe the reaction, and compare the results to the chart provided in the kit.
The Nitric Acid Test
Gold is a noble metal which means its resistant to corrosion, oxidation and acid. To perform this test, rub your gold on a black stone to leave a visible mark. Then apply nitric acid to the mark. The acid will dissolve any base metals that aren't real gold.
For testing gold jewelry or bullion, you have to rub (or scratch) it across the porcelain tile/ceramic plate. Then you analyze the color of the produced streak. If the gold is real, then it should show a golden, yellow color. If you see a black streak instead, then you have pyrite or fake gold.
Aqua regia, a mixture of one part nitric acid and three parts hydrochloric acid, is used to test higher karat purity through the process of comparison and elimination.
Drop a small amount of liquid nitric acid on that scratch and wait for a chemical reaction. Fake gold will immediately turn green where the acid is. Gold-over-sterling silver will become milky in appearance. Gold will not react to the nitric acid.
Apply concentrated acid to the metal. Brass will react with acids and gold will not. If you see bubbling or discoloration where the acid is applied, your piece is brass. If there is no change after applying the acid, you have gold.